Apparatus and methods for signal generation, reception, and self-calibration

ABSTRACT

Apparatus and methods for signal generation, reception, and calibration involving quadrature modulation and frequency conversion. Embodiments of the present invention provide extremely wide bandwidth, high spectral purity, versatility and adaptability in configuration, and ease of calibration, and are particularly well-adapted for use in integrated circuitry.

This is a Continuation of U.S. patent application Ser. No. 15/413,447filed Jan. 24, 2017.

BACKGROUND

Generating broadband quadrature-modulated signals presents a number ofchallenges in achieving wide bandwidth and spectral purity, in shapingthe waveform, in eliminating spurious components and non-linearitiesfrom the delivered signal, and in calibrating the quadrature modulation.Receiving and demodulating the signals presents similar challenges,particularly in cases of frequency conversion relating to imagerejection and local oscillator (LO) leakage at the intermediatefrequency (IF). Another issue involves quadrature imbalance at the localoscillator, where imbalances occurring at different points in thetransmit/receive path are typically inseparable and are therefore notreadily correctible. Thus, it would be desirable to have apparatus andmethods for generating and receiving quadrature-modulated signals havingnot only wide bandwidth and spectral purity, but also featuring ease ofcalibration and rejection of undesirable artifacts in the signal. Thisgoal is met by embodiments of the present invention.

SUMMARY

An embodiment of the present invention provides extremely wide-bandsignal-generating apparatus featuring multiple signal synthesizers andmultiple quadrature modulators having independently-selectableconfigurations for flexible interconnections. Apparatus according tothis embodiment allows convenient combination and isolation of differentsections to enable convenient characterization of spectral componentsand filters for optimizing performance and rejection of spurious signalartifacts.

Another embodiment of the present invention provides quadraturemodulators having internal digital filters to compensate for thefrequency-dependencies of low-pass anti-aliasing filters.

A further embodiment of the present invention provides digitalpre-processing apparatus for conditioning an input waveform to signalgeneration apparatus as disclosed herein.

Other embodiments of the present invention provide methods forself-calibration of analog and digital components of signal generatingand receiving apparatus as disclosed herein.

Embodiments of the present invention are particularly well-suited tobeing incorporated within integrated circuits.

Further advantages offered by embodiments of the present inventioninclude the ability for configurations to be adapted on the fly, and tobe adaptively optimized according to the specific environment andoperational settings. Embodiments of the present invention can thus beoptimized in the various degrees of freedom (e.g. per frequency) forperformance, spur rejection, interference resiliency, signal-to-noiseratio, bit error rate, and so forth.

It is understood that the present invention is not limited to theparticular area of Radar and that embodiments of the invention are alsoapplicable to other areas of the microwave signal field; including butnot limited to: communications; radio frequency (RF) imaging; multipleinput-multiple output (MIMO) communications and phased arrays;sensor-based applications (e.g. material analysis/monitoring); and testequipment implementation, such as vector network analyzers (VNA).

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter disclosed may best be understood by reference to thefollowing detailed description when read with the accompanying drawingsin which:

FIG. 1A illustrates a signal generator with pre-corrected digital inputsaccording to an embodiment of the present invention.

FIG. 1B illustrates a sideband selector switch for the signal generatorof FIG. 1A, according to a related embodiment of the present invention.

FIG. 2 illustrates a signal generator according to an embodiment of thepresent invention.

FIG. 3 illustrates a multiple signal generator according to anembodiment of the present invention.

FIG. 4 illustrates a transceiver according to an embodiment of thepresent invention.

FIG. 5 illustrates a quadrature receiver according to an embodiment ofthe present invention.

FIG. 6 illustrates a multistatic radar apparatus according to anembodiment of the present invention.

FIG. 7 illustrates a 3-channel MIMO transceiver according to anembodiment of the present invention.

FIG. 8 illustrates a spectral component measurement arrangement at theoutput of the signal generation block according to an embodiment of thepresent invention.

FIG. 9 illustrates a receiver-assisted spectral component measurementarrangement according to an embodiment of the present invention.

FIG. 10A illustrates a symmetrized receiver-assisted spectral componentmeasurement arrangement for characterization of a first quadraturemodulation block according to the present invention.

FIG. 10B illustrates a symmetrized receiver-assisted spectral componentmeasurement arrangement for characterization of a second quadraturemodulation block according to the present invention.

FIG. 11 illustrates a multi-module referenced based scaling arrangementaccording to an embodiment of the present invention.

FIG. 12 is a flowchart of a method of calibrating a signal generatoraccording to an embodiment of the present invention.

For simplicity and clarity of illustration, elements shown in thefigures are not necessarily drawn to scale, and the dimensions of someelements may be exaggerated relative to other elements. In addition,reference numerals may be repeated among the figures to indicatecorresponding or analogous elements.

DETAILED DESCRIPTION

FIG. 1A illustrates a signal generator 100 with pre-corrected digitalinputs 181 and 183 according to an embodiment of the present invention.

In a quadrature modulation block 101, digital-to-analog converters (DAC)103 and 107, respectively, receive digital inputs 181 and 183 and sendanalog signals corresponding thereto to anti-aliasing low-pass filters(LPF) 105 and 109, respectively. Digital input 181 is a pre-correctedin-phase input Ic, whereas digital input 183 is a pre-correctedquadrature input Q_(C). Anti-aliasing low-pass filters 105 and 109 inturn output signals to multiplicative mixers (“mixers”) 111 and 113,respectively. A 90° splitter 115 receives a synthesized frequency from asynthesizer 121 and outputs two signals which are 90° out of phase, withthe signal to mixer 113 lagging 90° behind the signal to mixer 111. Themixed outputs from mixer 111 and mixer 113 are input to a summing unit117.

The output from quadrature modulation block 101 is input to a switch133A, which can be selectably switched to pass the direct output ofquadrature modulation block 101 or the output of quadrature modulationblock 101 mixed by a mixer 131 with a synthesized frequency from asynthesizer 123.

Various embodiments of the invention feature switches configured in amanner similar to that of switch 133A. Certain embodiments of theinvention provide that these switches be independently selectablyswitchable. Independent switchability according to these embodiments ofthe invention not only provides versatility in configuring apparatus,but also provides benefits in calibration of the apparatus, as detailedbelow.

Quadrature modulation typically suffers from spurious image-frequencysignal and from local oscillator feed-through. These imperfections canbe significantly reduced by signal pre-compensation in the digitaldomain. The setting of the pre-compensation or pre-correctioncoefficients requires a feedback mechanism allowing the measurement ofthe above spurious signals.

Therefore, an embodiment of the present invention provides forpre-correction as follows. A numerically-controlled oscillator (NCO) 141receives a frequency signal 143 to set the frequency f of theoscillator, and an initial phase signal 143 to set the initial phase φ₀.Numerically-controlled oscillator 141 outputs two signals, a sine wave147 sin (f, φ₀) and a cosine wave 149 cos (f, φ₀), which are input to acomplex multiplier 151, whose other inputs are an in-phase data stream153 I_(data) (k) and a quadrature data stream 155 Q_(data) (k). Thecomplex product outputs of complex multiplier 151 are a desired in-phasedata wave 157 I and a desired quadrature data wave 159 Q. However, inorder to compensate for effects such as amplitude imbalance ofquadrature modulation to be performed by quadrature modulation block101, a pre-correction is needed, which is furnished by a matrixmultiplier 161, containing filters 163, 165, 167 and 169 for singlesideband (SSB) rejection. In addition, matrix multiplier 161 alsocorrects for local oscillator leakage with direct current offsets I_(DC)and Q_(DC) into summing mixers 177 and 175, respectively.

Furthermore, in accordance with an embodiment of the present invention,digital filters 163, 165, 167, and 169 feeding into summing mixers 171and 173, respectively, are incorporated into matrix multiplier 161 tocompensate for the frequency-dependencies of anti-aliasing low passfilters 105 and 109. The result, as previously noted, are pre-correctedin-phase input 181 I_(C) and pre-corrected quadrature input 183 Q_(C).

FIG. 1B illustrates a sideband selector configuration switch 133Baccording to a related embodiment of the present invention. Sidebandselector configuration switch 133B selectively switches between thedirect output of quadrature modulation block 101 and either the uppersideband of the output of quadrature modulation block 101 mixed viamixer 131 with the output of synthesizer 123, or the lower sidebandthereof, as passed by an upper sideband filter 135 or a lower sidebandfilter 137, respectively.

In the above descriptions, transmission signal generation is a hybridconfigurable one/two conversion process as illustrated in FIG. 1A. Thedifferent states reached under the topology depend on the setting ofswitch 133A and are as follows:

-   -   Direct conversion based on a frequency synthesizer 121, which is        directly modulated by wide-band quadrature modulator block 101;    -   Double conversion operation based on mixing between the output        of quadrature modulator block 101 with synthesizer 123.

This architecture inherently features an extremely wide frequencycoverage (DC to 10 s GHz) while maintaining low spurious signal content.In certain cases the synthesizer frequency range is increased by digitaldividers. In these cases, for noise minimization and stability, it maybe of interest to have the synthesizers operate at differentfrequencies. Digitally divided signals, however, typically have highspurious harmonic content. Operation over a multi-octave frequency rangenormally requires complicated re-configurable filters and filter banksto suppress these spurious signals. By heterodyne down-conversion of thedirect modulated signal, wide frequency coverage can be achieved withthe spurious signals lying out-of-band.

As the frequency coverage requirement widens, so does the coveragerequirement from the synthesizers and direct modulators. Employing bothdirect and double conversions may relax the above requirement. Forexample, a quadrature modulator covering the range 4-8 GHz may be mixedwith an additional 8-12 GHz synthesizer in order to cover the DC-4 GHzrange, and with a 12-16 GHz synthesizer in order to cover the 8-12 GHzfrequency range. Higher frequencies may be covered by usingup-conversion rather than down-conversion.

Another benefit provided by embodiments of the present invention is thecapability of arbitrarily modulating a wide-band waveform (as wide asthe baseband) at any frequency within the frequency coverage. Thispermits the use of modulation schemes such as chirp/pseudo-random bitsequence (PRBS) for pulse compression in radar applications,communication constellations, and so forth.

Further use of the arbitrary digital modulation provided by embodimentsof the present invention allows a fine-frequency offset in the digitaldomain. This permits coarser frequency steps in the synthesizers,improving their phase noise performance for the same frequencyresolution requirement.

Another benefit provided by embodiments of the present invention is theability to reach a certain output frequency via several differentconfigurations. In a non-limiting example, by stepping the synthesizerto a higher frequency and correspondingly stepping the basebandfrequency to a lower frequency the output frequency is unchanged. Thisis instrumental in producing a coherent frequency coverage across allsynthesizer frequencies, even though it does not retain a specific phaseover frequency change.

FIG. 2 illustrates a signal generator according to another embodiment ofthe present invention, where a second quadrature modulation block 203 isutilized to directly modulate synthesizer 123 to create the localoscillator for the second conversion. This enables a tradeoff ofquadrature modulation imbalance versus phase noise to attain arbitraryfrequency in generating the local oscillator for the conversion node.

FIG. 3 illustrates a multiple signal generator according to anembodiment of the present invention. Frequency synthesizer 301 feedsquadrature modulation blocks 303 and 305, and frequency synthesizer 351feeds quadrature modulation blocks 353 and 355. Selector switches 311,331, 361, and 381 operate as previously described for selector switch133A (FIG. 1A), and selectably switch between the direct output ofquadrature modulation blocks 303, 305, 353, and 355 respectively, andoutputs of mixers 313, 333, 363, and 383, respectively, all of whichreceive input from frequency synthesizer 391.

As previously noted, various embodiments of the present inventionprovide selector switches 311, 331, 361, and 381 to be independentlyswitchable.

The arrangement illustrated in FIG. 3 is useful in Radar communicationsystems where there is a need for multiple microwave signals inparallel. Non-limiting examples of such needs include:

-   -   Simultaneous generation of transmit signal and of a receive        local oscillator signal;    -   Generation of multiple transmit signals in multiple        input-multiple output (MIMO) and phased/true delay array        systems; and    -   Generation of sine and cosine local oscillator signals of        quadrature down conversion.

For example, by digitally modulating the transmit signal and the receivelocal oscillator signal in a short range frequency-modulated continuouswave (FM-CW) radar system one can introduce an intentional frequencyoffset so as to avoid handling near-DC signals (see FIG. 4). An inherenttrait of this architecture is that several direct conversion blocks andheterodyne converters may be fed from the same synthesizers, therebynaturally meeting the aforementioned need. This allows phase trackingbetween different microwave signals, as well as tracking of the phasenoise.

Another advantage of this architecture is the distribution of agenerated signal among many nodes, such as transmissionantennas/receivers etc. This enables applications such as “MultistaticRadar” (see below).

Further embodiments of the present invention provide multiplesynthesizers (as in FIG. 3), some of which are modulated and some arenot, so as to simultaneously generate multiple signals at arbitrarilyspaced frequencies.

FIG. 4 illustrates a transceiver according to an embodiment of thepresent invention. A frequency synthesizer 401 feeds quadraturemodulator blocks 403 and 405 having selector switches 411 and 431respectively, which select between direct output from the quadraturemodulator blocks and the outputs of mixers 413 and 433, respectively,both of which receive input from a frequency synthesizer 407. The outputof selector switch 411 feeds into an amplifier 451, which in turn feedsan antenna switch/circulator 453 to an antenna 455 for transmission.Signals received from antenna 455 (such as by reflections of thetransmitted signal) are fed to a mixer 457, which receives input fromswitch 431. Output of mixer 457 feeds to an anti-aliasing low-passfilter 459 and thence to an analog-to-digital converter 461 (ADC).

By modulating quadrature modulation blocks 403 and 405, fed by the samesynthesizer 407 with a frequency shift, both the transmit signal andlocal oscillator drive for an arbitrary intermediate frequency (IF)receiver are produced. The received signal is down-converted to anintermediate frequency corresponding to the offset of the modulationfrequency between quadrature modulation blocks 403 and 405.

Another example of arbitrary waveform modulation-based receiver localoscillator generation is a modulation with a pseudo-random binarysequence (PRBS) modulation, for a spread-spectrum radar.

A further example of an arbitrarily-configurable demodulation ismulti-tone demodulating. Such a configuration is useful in thesimultaneous measurement of several spectral components, e.g. bydown-converting them to distinct intermediate frequencies. Both theamplitudes and phases of the spectral components may be measured.

The above capability of the signal generator for attaining an outputfrequency in several configurations, enables relating measurementsacross the entire frequency range, i.e. including local oscillator andmeasured path phase. According to a related embodiment, this is achievedby overlapping measurements between different local oscillatorfrequencies, where the baseband frequencies are adjusted to account forthe local oscillator frequency offset between the measurements. Thisphase-related measurement differs from the common practice in the art,where, as the local oscillator is tuned over the coverage range,unaccounted-for phase changes occur. Retaining the relative phaseaccording to this embodiment is instrumental in characterizingnon-linear parameters in a vector network analyzer (VNA) embodiment ofthe present invention.

FIG. 5 illustrates a quadrature receiver according to an embodiment ofthe present invention. A switch 511 and a switch 531 are ganged togetherby a common selector 533, to generate a 0° local oscillator 541 and a90° local oscillator 543, which feed mixers 561 and 563, respectively,to convert a signal received by an antenna 555, which is amplified by anamplifier 551. The two intermediate frequency signals are fed intoanti-aliasing low-pass filters 571 and 575, respectively, to bedemodulated by analog-to-digital converters 573 and 577, respectively.

The configuration illustrated in FIG. 5 allows the generation of a 90°split over a wide frequency range, as opposed to conventional analogtechniques, and without introducing substantial spurious harmoniccontent, which occurs when using digital dividers.

According to related embodiments of the invention, calibrationtechniques can be used to adjust the relative phase and amplitudebetween the quadrature channels. In non-limiting examples: measuring thephase and amplitude between the in-phase (I) and quadrature (Q)components of the down-converted continuous wave signal; simultaneouslymeasuring the phase and amplitude on several signals; andcross-correlation measurements between the I and Q arms.

FIG. 6 illustrates a multistatic radar apparatus according to anembodiment of the present invention. In many cases it is desirable for agenerated signal to be distributed among many nodes, such astransmission antennas/receivers, and so forth.

FIG. 7 illustrates a 3-channel multiple input-multiple output (MIMO)transceiver according to an embodiment of the present invention. In thisembodiment, the above-described coherent arbitrary modulation topologyis used in conjunction with parallelism (i.e. all quadrature modulationblocks are fed by the same synthesizer and are coherent to each other).This configuration enables active beamforming such as in the context ofphased-array antennas. Current implementations are usable principally innarrow-band arrays, where carrier frequencies reach the microwave regimeand analog delay-induced phase shifts are used. This embodiment of thepresent invention provides true beam-forming by digital relative delaymeans. Beam-forming is achieved by baseband modulation of coherentchannels relative to each other, and does not hinder the broad bandnature of the transceiver array. In addition, this embodiment providesease of implementation with digital accuracy. Steering resolution andphase coherence are very precise since the relative phase attainable atany baseband frequency is practically arbitrary, as it is limitedprincipally by digital-to-analog converter resolution.

Calibration

Calibration plays a significant role, where quadrature modulationimbalance, local oscillator leakage and the response of the receiver andtransmitter paths comprise fundamental factors in attaining the requiredperformance of a transceiver.

Quadrature modulation imbalance and local oscillator leakagecalibrations are typically performed by a minimization of mixingproducts after passage through a broadband envelope detector. Thequadrature modulator is subjected to modulation by complex sine wave atfrequency f_(BB). At the output of the envelope detector, the detectedpower fluctuates at frequencies associated with the frequency offsetbetween the desired signal and the spurious signals (either 2 f_(BB) forthe quadrature modulation image or f_(BB) for the local oscillatorleakage). The power fluctuations are typically measured by ananalog-to-digital converter (ADC). It is important to note that if ahigh f_(BB) is used then a high speed ADC is needed in order to captureand quantify the power fluctuations (the ADC bandwidth needs to be atleast twice the baseband bandwidth in order to capture both spectralcomponents).

Current techniques suffer from inherent difficulties associated withspurious signals and mixing products which fall on the to-be-measuredquantities. As an example, mixing products from 2f_(signal)−2f_(LO) fallon the to-be-measured frequency associated with the quadrature modulatedimage: f_(signal)−f_(image). Thus, the measurements are not independent.Embodiments of the invention facilitate the calibration for quadraturemodulation imbalance and local oscillator leakage, without increase inarchitectural complexity.

The corrective action for compensation of quadrature modulationimbalance and local oscillator leakage are well known in the art. Thequadrature modulation imbalance compensation involves pre-multiplyingthe I and Q components by a matrix of correction coefficients. Thecompensation of local oscillator leakage typically involves adding DCcoefficients to the I and Q components. The difficult part of thisprocedure is determining which coefficients' values to use. Thisinvolves a feedback measurement of the strength of the image andspectral components of the local oscillator leakage.

FIG. 8 illustrates a spectral component measurement arrangement at theoutput of the signal generation block according to an embodiment of thepresent invention. Here, two quadrature modulation blocks are fed by asingle, common, synthesizer. A method of measuring the image or localoscillator leakage is by placing the second synthesizer—used to convertthe signal to the baseband—at a frequency offset relative to thespectral component of interest.

To measure the image, situated at f_(image)=f_(Sa)−f_(IQal), placing thesecond synthesizer at f_(Sb)=f_(image)−f_(IF) which will be, afterf_(IF) conversion, linear in the original image magnitude. In order toreach the desired frequency at the output of the secondsynthesizer—driving the conversion of the output of the quadraturemodulation block—fine frequency selection may be facilitated byeither/or both the utilization of a fractional N synthesizer and anquadrature modulation of the synthesizer output. Only a single channel(one quadrature modulator, two synthesizers) is needed for the abovescheme.

FIG. 9 illustrates a receiver-assisted spectral component measurementarrangement according to an embodiment of the present invention.

FIG. 10A illustrates a symmetrized receiver-assisted spectral componentmeasurement arrangement for characterization of a first quadraturemodulation block according to the present invention.

FIG. 10B illustrates a symmetrized receiver-assisted spectral componentmeasurement arrangement for characterization of a second quadraturemodulation block according to the present invention.

Baseband Filter Characterization

Baseband filter characteristics may vary at production. In the case ofintegrated circuit implementation, the filter bandwidth and shape maydepend on process, temperature and voltage. The characteristics ofbaseband filters in the transmit and receive chains may affect systemperformance regarding signal-to-noise ratio, inter-symbol interference,power flatness, mask conformity, and so forth. It is thus desirable tocharacterize the filters and compensate for their deviation from desiredcharacteristics. Examples of compensation include directly adjusting thefilter and performing digital compensation.

The hardware architecture of embodiments of the present inventionfacilitates measurement of filter characteristics without furtherincreasing complexity.

To characterize the transmit filter, the f_(BB) is scanned throughoutthe range of interest. For each f_(BB) the synthesizer's frequencies(f_(sa), f_(sb)) are adjusted such that the resulting intermediatefrequency is constant; thus avoiding the receive filter responsevariation (when measuring at different intermediate frequencies perf_(BB)).

The receiver can be tuned to a frequency corresponding to an aliasedfrequency ±f_(BB)+N·f_(sample) (where f_(sample) is thedigital-to-analog converter sampling frequency). By doing so, the lowpass filter in the transmit path can be characterized beyond the Nyquistfrequency of the digital-to-analog converter.

Embodiments of the invention as described above and depicted in FIG. 8and FIGS. 10A and 10B illustrate two similar schemes for scanning thebaseband frequency as described above, by digitizing the output of thesignal generation block.

Measuring the receiver filter is conceptually similar to the aboveschemes, but benefits from prior knowledge of the transmitter filterresponse: by knowing the response of the transmission filter, thequadrature modulation frequency can be tuned to scan the frequencies ofthe receiver filter. Alternatively, it is possible to measure thereceiver filter separately without first characterizing the transmissionfilter. To do so, the quadrature modulation is held at a constantfrequency (so as to not incur response variation) and the receiverfrequencies are scanned by tuning the synthesizer's frequencies.

The intermediate frequency can be tuned beyond the Nyquist frequency ofthe analog-to-digital converter so that the receive anti-alias low-passfilter reacts to the input intermediate frequency, while the digitizedoutput is at an aliased frequency ±f_(BB)+N·f_(sample) (where f_(sample)is the analog-to-digital converter sampling frequency. By doing so, thelow pass filter in the receive path can be characterized beyond theNyquist frequency of the analog-to-digital converter.

Self-Characterization of Phase Noise

Digitization of the first synthesizer, down converted by the secondsynthesizer, allows characterizing the relative phase noise between thetwo synthesizers. This measurement can be used for either self-testpurposes or for performance optimizations, such as setting thephase-locked loop parameters so as to optimize the phase noise. Anexample of such parameter is the setting of the charge pump current inthe phase detector.

FIG. 11 illustrates a multi-module referenced based scaling arrangementaccording to an embodiment of the present invention.

FIG. 12 is a flowchart 1200 of a method of calibrating a two-synthesizersignal generator according to an embodiment of the present invention. Ina step 1201 the first frequency synthesizer is set to the desired testfrequency. In a step 1203 an outer loop begins, in which the firstnumerically-controlled oscillator is set to the desired test frequencyoffset. In a step 1205, the second frequency synthesizer and the secondnumerically-controlled oscillator are set to obtain the desiredreceiving intermediate frequency.

In a step 1207 an inner loop begins for configuring a set of quadraturemodulation imbalance correction coefficient values, and in a step 1209an imbalance-related magnitude is measured. At a decision point 1211, ifthe coefficient set is not exhausted, the method returns to step 1207.Otherwise, if the set is exhausted, the loop beginning in step 1207exits and the method proceeds to a step 1213, in which optimalcorrection coefficients are calculated.

At a decision point 1215, if the first numerically controlled oscillatorfrequencies are not exhausted, the method returns to step 1203.Otherwise, if the frequencies are exhausted, the loop beginning in step1203 exits, and the method concludes with a step 1217, in which theoptimal frequency-dependent correction coefficients are calculated.

What is claimed is:
 1. A system for frequency-dependent characterizationand compensation of an element of a transceiver, the system comprising:a first synthesizer; a first pair of digital-to-analog converters(DACs), each DAC connected to a respective anti-alias filter; a firstquadrature modulator, connected to the first synthesizer and to outputsof the anti-alias filters; a second synthesizer; a mixer, connected tothe output of the first quadrature modulator and to the secondsynthesizer; a receive filter connected to the output of the mixer; ananalog-to-digital converter (ADC) connected to the output of the receivefilter; and a unit for characterizing the output of the ADC at aselected frequency; wherein the digital inputs to the first pair of DACsand the configuration of the second synthesizer are selected so as to:obtain a characterization of a desired frequency component; and from thecharacterization to apply a compensation for a response of the element.2. The system of claim 1, wherein the second synthesizer comprises asecond pair of DACs, each followed by an anti-alias filter and a secondquadrature modulator connected to the second synthesizer and to outputsof the anti-alias filters.
 3. The system of claim 1, wherein the systemis configured to characterize a plurality of digital inputs to the firstpair of DACs and configurations of the second synthesizer.
 4. The systemof claim 1, wherein the element is an anti-alias filter.
 5. The systemof claim 1, wherein the element is the receive filter.
 6. The system ofclaim 1, wherein the element is the first quadrature modulator.
 7. Thesystem of claim 1, wherein the digital inputs to the first pair of DACsand the configuration of the second synthesizer are selected so as toobtain a plurality of characterizations.
 8. The system of claim 1,wherein the digital inputs to the first pair of DACs and theconfiguration of the second synthesizer are selected so as to apply aplurality of compensations.
 9. The system of claim 1, wherein thecompensation is applied by tuning the element.
 10. The system of claim1, wherein the compensation is applied by means of digital compensation.11. The system of claim 4, wherein the characterization and compensationfor an anti-alias filter are beyond a Nyquist frequency of a DAC. 12.The system of claim 5, wherein the characterization and compensation forthe receive filter are beyond a Nyquist frequency of the ADC.